The present invention relates to solid oxide fuel cells (SOFCs), particularly to SOFCs utilizing cobalt iron based electrodes, and more particularly to a method for fabricating SOFCs wherein a buffer layer of doped-ceria is deposited by a colloidal spray deposition technique intermediate the zirconia electrolyte and the cobalt iron manganese based electrode, whereby the power density of the SOFCs is increased 2-3 times over conventionally known cells.
(Solid Oxide Fuel Cells) SOFCs are solid state electrochemical devices that convert chemical fuels directly to electricity. Because no combustion is involved, SOFCs are not limited by the Carnot cycle. As a result, SOFCs can have efficiency much higher than conventional power generation devices. The use of SOFCs would result in fuel economy and lower carbon emissions. For these reasons, SOFCs are currently of high interest for clean and efficient electricity generation for stationary and transportation applications.
However, despite many successful demonstrations by Siemens Westinghouse, the SOFC commercialization is still not envisioned for a close future because of the excessive high cost. Such a high cost comes from the inherent low power density of the Westinghouse tubular design (300 mW/cm2 at 1000° C.) and particularly from the manufacturing cost due to the expensive processing techniques. An alternative to the Westinghouse tubular fuel cell is the planar design that has potentially higher power density and can be operated at lower temperatures, 800° C. or lower. The operation at intermediate temperatures makes possible the use of cheaper materials such as the metal interconnect and while putting less constraints on the materials and the gas manifolding system. However, planar fuel cell technology is less mature and a number of issues must be resolved before possible commercialization. In particular, although certain alloys can be used at 800° C., they still tend to oxidize severely after several hundred hours of operation. Therefore, there is strong interest to further decrease this operating temperature to below 700° C. Unfortunately, most of current planar SOFCs loss rapidly performance when temperature decreases. For example, the Honeywell (ex Allied Signal) fuel cell power density drops from 650 mW/cm2 at 800° C. to 350 mW/cm2 at 700° C., that of the European ECN fuel cell drops from 610 mW/cm2 at 800° C. to 270 mW/cm2. Thus, current SOFCs are not suitable for operation at temperature below 700° C. It is noted that very high power density SOFCs, up to 2 W/cm2 at 800° C. and 1 W/cm2 at 700° C., have been reported by Lawrence Berkeley National Lab and the University of Utah. However, the measurement conditions are unclear. Recently, we have shown that certain testing configurations can yield largely over estimated power density. When single cells are tested in the asymmetric configuration, i.e. where one of the electrodes is significantly larger than the other one and the power density is then normalized to the smaller electrode area, such artificially increased the power density by the favorable normalization effect. The asymmetric cell configuration does not correspond to the actual fuel cell stack operating condition. Therefore, all comparisons should not take into account data obtained using asymmetric configuration.
It is well known that (La,Sr)(Co,Fe)O(LSCF)is a much better cathode material than the conventional (La,Sr)MnO electrode. Unfortunately, the cobalt iron based electrode tends to react with the zirconia electrolyte, causing rapid long-term degradation. A buffer layer of doped-ceria has been proposed to avoid the direct contact of the LSCF electrode with the zirconia electrolyte. Although the concept is interesting, the reduction to practice has not been successful because of the lack of an adequate technique to deposit the buffer coating. For instance, doped-ceria has been deposited on a zirconia electrolyte using sputtering or conventional screen-printing techniques. Cracking of the buffer layer has been observed because of the difference in thermal expansion coefficients between the two layers. No significant performance improvement has ever been reported.
The present invention provides a solution to the above-mentioned problem regarding the formation of a buffer layer between an LSCF electrode and a zirconia electrode. This is accomplished by depositing a doped-ceria buffer layer using a colloidal spray deposition (CSD) technique such as described and claimed in U.S. patent No. 6,358,567, filed Apr. 16, 1999, entitled “Colloidal Spray Method for Low Cost Thin Coating Deposition”, whereby the doped-ceria buffer layer is deposited on the zirconia layer without cracking, and then the LSCF electrode is subsequently deposited on top of the buffer layer using the same CSD technique. Thus, the power density of the SOFCs formed by the method of the present invention is increased by 2-3 times over that of the conventional fabricated SOFCs.